Gluon Shadowing and Heavy Flavor Production off Nuclei

TL;DR

The paper develops a light-cone dipole framework to quantify gluon shadowing in heavy-flavor production off nuclei, revealing strong process-dependent shadowing arising from ccbarG configurations and nonperturbative interactions. By classifying ccbar production into color singlet and octet states and deriving associated gluon-radiation amplitudes, it shows shadowing is larger than DIS and grows with energy, while remaining sensitive to the ccbar size and quantum numbers. The authors compute shadowing for open charm at RHIC and LHC, find significant suppression with little centrality dependence, and also discuss higher-twist effects and final-state absorption at intermediate energies. The LC-dipole approach unifies the treatment of shadowing and absorption, offering predictions testable by HERA-B, RHIC, and LHC experiments and highlighting the potential to constrain the gluon density in nuclei.

Abstract

Gluon shadowing which is the main source of nuclear effects for production of heavy flavored hadrons, remains unknown. We develop a light-cone dipole approach aiming at simplifying the calculations of nuclear shadowing for heavy flavor production, as well as the cross section which does not need next-to-leading and higher order corrections. A substantial process dependence of gluon shadowing is found at the scale of charm mass manifesting a deviation from QCD factorization. The magnitude of the shadowing effect correlates with the symmetry properties and color state of the produced c-cbar pair. It is about twice as large as in DIS, but smaller than for charmonium production. The higher twist shadowing correction related to a nonzero size of the c-cbar pair is not negligible and steeply rises with energy. We predict an appreciable suppression by shadowing for charm production in heavy ion collisions at RHIC and a stronger effect at LHC. At the same time, we expect no visible difference between nuclear effects for minimal bias and central collisions, as is suggested by recent data from the PHENIX experiment at RHIC. We also demonstrate that at medium high energies when no shadowing is possible, final state interaction may cause a rather strong absorption of heavy flavored hadrons produced at large x_F.

Figures (8)

• Figure 1: Perturbative QCD mechanism of charm production production in a gluon-nucleon collision. Only the lowest $\bar{c}c$ Fock component of the gluon is taken into account.
• Figure 2: Perturbative QCD mechanism for production of a $\bar{c}c$ pair and a gluon in a gluon-nucleon collision. The upper blob includes different attachments of the gluons as is depicted in Fig. \ref{['graphs']}.
• Figure 3: Born graph for scattering of a gluon with color indexes $b,\ b'$ on a $\bar{c}c$ pair in color-octet state with indexes $g,\ g'$.
• Figure 4: Ratio of gluon densities $R_G(Au/p)=G_{Au}(x_2)/195\,G_{p}(x_2)$ for color octet-octet and singlet-octet states $(\bar{c}c)-G$ (dashed curves). The averaged gluon shadowing is depicted by solid curve.
• Figure 5: Nuclear effects for open charm production in $p_W$ collisions at $900\,\hbox{GeV}$ in the laboratory frame. The contribution of gluon shadowing is shown by dashed curve. The solid curve represents the full shadowing effect including shadowing of charmed quarks in the nucleus. Possible medium effects including antishadowing and EMC-suppression for gluons eks are also added and the result is represented by the dotted curve.